Heretofore, many compounds, each of which has a crystal structure of garnet (hereinafter, also referred to as a garnet structure), have been known. Natural garnet is a silicate mineral, and those with high transparency have been valued as jewelry since ancient times, and sand of the garnet is industrially used as abrasive.
Famous garnet includes almandine (Fe2+3Al2(SiO4)3), grossular (Ca3Al2(SiO4)3), and andradite (Ca3Fe3+2(SiO4)3). Moreover, the famous garnet includes pyrope (Mg3Al2(SiO4)3, spessartine (Mn3Al2(SiO4)3, uvarovite (Ca3Cr2(SiO4)3, and the like.
Here, a compound represented by Y3Al2(AlO4)3, (which is hereinafter also referred to as YAG), is an artificial mineral synthesized based on the garnet, and is widely known as a name of yttrium aluminum garnet. Then, YAG is used for the purpose of a solid-state laser, translucent ceramics, a phosphor and the like (for example, refer to Non-Patent Literature 1). Moreover, it is known that the YAG includes many modification examples. Typical modification examples of YAG include Tb3Al2(AlO4)3 (for example, refer to Patent Literature 1), Y3Ga2(AlO4)3 (for example, refer to Non-Patent Literature 1), Y3Mg2(AlO4)(SiO4)2 (for example, refer to Patent Literature 2) and the like.
Here, the phosphor stands for a compound that emits fluorescence by being given a stimulus such as ultraviolet excitation. Then, extranuclear electrons of a specific atom that composes the compound are excited by the ultraviolet ray and the like, and a difference in energy level is emitted as visible light when the extranuclear electrons return to a ground state thereof. For example, rare earth ions and transition metal ions (Ce3+, Tb3+, Eu3+, Mn2+, Mn4+, Fe3+, Cr3+ and the like), each of which functions as a emission center, are contained in such a compound as YAG, whereby the phosphor is obtained.
Then, a phosphor having a garnet structure, such as a YAG:Ce phosphor activated by Ce3+ and a YAG:Tb phosphor activated by Tb3+, (which is hereinafter also referred to as a garnet-type phosphor), is known as a high-efficiency phosphor. Then, the garnet-type phosphor as described above is used in numerous light-emitting devices (for example, refer to Patent Literatures 3 and 4 and Non-Patent Literature 1).
Note that the garnet-type phosphor activated by Ce3+ is characterized in, at a time of being irradiated with a corpuscular ray or an electromagnetic wave, being excited and emitting ultrashort afterglow visible light from blue green through green and yellow to red (for example, refer to Non-Patent Literature 1 and Patent Literature 2). Meanwhile, a YAG:Eu phosphor activated by Eu3+ is also known, and study thereof as a red phosphor for a plasma display device (PDP) is made (for example, refer to Non-Patent Literature 1).